We present the results of the γ-ray flux distribution study on the brightest blazars which are observed by the Fermi-LAT. We selected 50 brightest blazars based on the maximum number of detection reported in the LAT third AGN catalog. We performed standard unbinned maximum likelihood analysis on the LAT data during the period between August 2008 and December 2016, in order to obtain the average monthly flux. After quality cuts, blazars for which at least 90% of the total flux was survived were selected for the further study, and this includes 19 FSRQs and 19 BL Lacs. The Anderson-Darling and χ 2 tests suggest that the integrated monthly flux follow a log-normal distribution for all sources, except for three FSRQs for which neither a normal nor a log-normal distribution was preferred. A double log-normal flux distribution tendency were observed in these sources, though it has to be confirmed with improved statistics. We also found that, the standard deviation of the lognormal flux distribution increases with the mean spectral index of the blazar, and can be fitted with a line of slope 0.24 ± 0.04. We repeat our study on three additional brightest unclassified blazars to identify their flux distribution properties. Based on the features of their log-normal flux distribution, we infer these unclassified blazars may be closely associated with FSRQs.We also highlight that considering the log-normal behavior of the flux distribution of blazars, averaging their long term flux in linear scale can largely under estimate the nominal flux and this discrepancy can propagate down to the estimation of source parameters through spectral modeling.

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We perform a detailed spectral study of a recent flaring activity from the Flat Spectrum Radio Quasar (FSRQ), 3C 454.3, observed simultaneously in optical, UV, X-ray and γ-ray energies during 16 to 28 August, 2015. The source reached its peak γ-ray flux of (1.9 ± 0.2) × 10 −05 ph cm −2 s −1 on 22 August. The time averaged broadband spectral energy distribution (SED) is obtained for three time periods, namely "flaring state"; covering the peak γ-ray flux, "post flaring state"; immediately following the peak flare and "quiescent state"; separated from the flaring event and following the post flaring state. The SED corresponding to the flaring state is investigated using different emission models involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) mechanisms. Our study suggests that the X-ray and γ-ray emission from 3C 454.3 cannot be attributed to a single emission mechanism and instead, one needs to consider both SSC and EC mechanisms.Moreover, the target photon energy responsible for the EC process corresponds to an equivalent temperature of 564 K, suggesting that the flare location lies beyond the broad line emitting region of the FSRQ. SED fitting of the other two flux states further supports these inferences.

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Energy dependent phase lags reveal crucial information about the causal relation between various spectral components and about the nature of the accretion geometry around the compact objects. The time-lag and the fractional root mean square (rms) spectra of GRS 1915+105 in its heartbeat oscillation class/ρ state show peculiar behaviour at the fundamental and harmonic frequencies where the lags at the fundamental show a turn around at ∼ 10 keV while the lags at the harmonic do not show any turn around at least till ∼ 20 keV. The magnitude of lags are of the order of few seconds and hence cannot be attributed to the light travel time effects or Comptonization delays. The continuum X-ray spectra can roughly be described by a disk blackbody and a hard X-ray power-law component and from phase resolved spectroscopy it has been shown that the inner disk radius varies during the oscillation. Here, we propose that there is a delayed response of the inner disk radius (DROID) to the accretion rate such that r in (t) ∝ṁ β (t − τ d ). The fluctuating accretion rate drives the oscillations of the inner radius after a time delay τ d while the power-law component responds immediately. We show that in such a scenario a pure sinusoidal oscillation of the accretion rate can explain not only the shape and magnitude of energy dependent rms and time-lag spectra at the fundamental but also the next harmonic with just four free parameters.

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The work compiles a correlated study of a gravitational quasi equilibrium thermodynamic approach for establishing and signifying a unique behavior of the cosmological entropy and phase transitions in an expanding Universe. On the basis of prescribed boundary conditions for the cluster temperature a relation for the intra-cluster medium (ICM) of galaxy clusters has been derived. A more productive and signifying approach of the correlation functions used for galaxy clustering phenomena shows a unique behavior of the entropy change where a phenomenon known as the gravitational phase transition occurs. This unique behavior occurs with a symmetry breaking from mild clustering to low clustering and from mild clustering to high clustering which differs from a normal symmetry breaking in material sciences. We also derive results for the specific latent heat associated with the phase transitions of 3.20 T c and 0.55 T c for the mildly clustered phase to the low clustered phase and from the mildly clustered phase to the highly clustered phase, respectively.

In the present work the approach-thermodynamics and statistical mechanics of gravitating systems is applied to study the entropy change in gravitational clustering of galaxies in an expanding universe. We derive analytically the expressions for gravitational entropy in terms of temperature T and average density n of the particles (galaxies) in the given phase space cell. It is found that during the initial stage of clustering of galaxies, the entropy decreases and finally seems to be increasing when the system attains virial equilibrium. The entropy changes are studied for different range of measuring correlation parameter b. We attempt to provide a clearer account of this phenomena. The entropy results for a system consisting of extended mass (non-point mass) particles show a similar behaviour with that of point mass particles clustering gravitationally in an expanding universe.